"Females in the population had a very strong reaction to the red males and really preferred them as extra-pair partners," says Baldassarre, who carried out the research for his PhD.

The Australian red-backed fairy-wren is widely distributed across northern Australia and consists of two sub-species whose males have different coloured back plumage - red and orange.

Red birds dominate across northern Australia whereas the orange birds dominate down the east coast and there is an area in which they overlap.

But, a puzzling observation has been that in this overlap area, there are hybrid birds that seem to be genetically orange but have red plumage.

"Those orange birds are keeping most of their own genes but are taking on the genes for red plumage," says Baldassarre.

"So the red is slowly encroaching into the genetic background of that orange sub-species."

The question is, what is responsible for this asymmetrical hybridisation?

Paint it red

Baldassarre and colleagues proposed that the observation could be explained by sexual selection.

"There's something about those red birds that allows them to mate more successfully than the orange birds," says Baldassarre.

To test this, the researchers used a population of fairy-wrens in Queensland to carry out a "plumage manipulation experiment", in which they turned some orange birds into red ones.

"We took a bunch of these wild free-flying orange males and painted them with a red non-toxic marker, which was a really great simple way of turning orange birds red," says Baldassarre.

He and colleagues then released the newly-red males and monitored them through the breeding season to compare their mating success with that of the unpainted orange males.

They looked at the males' success both with their regular partners and with their flings.

While fairy-wrens do form pairs, they are famous for mating outside this.

"They have a very promiscuous mating system," says Baldassarre.

Reproductive success

The researchers found that while there was little difference in mating success within pair bonds, there was a big difference in success in mating outside their pair bond.

"When it came to extra-pair paternity, [the orange birds painted red] had around double the number of offspring of the normal orange males and so that led to much higher reproductive success," says Baldassarre.

Interestingly they did not find that naturally redder orange males had greater success at mating, Baldassarre thinks that even the reddest naturally-occurring orange birds were not red enough to compete with the naturally fully-red males.

They researchers propose the idea that the painted birds stimulated the females because they were new and stood out against the background.

"They'd never seen red males like that before. It was a totally new colour in the population," says Baldassarre.

Speciation

Baldassarre says the two sub-species of fairy-wren were once geographically isolated and probably accumulated colour differences during this time.

"They were on the path to becoming two separate species," he says.

But, he adds, once the glaciers retreated in Pleistocene, the birds were able to come back into contact with each other and had not been separated long enough for there to be a big cost to them hybridising.

"This extra-pair mating could be a hidden avenue that could work against speciation process," says Baldassarre.